Most birds can move their legs alternately to walk and run, and some birds are adept at climbing trees. Birds' agility on land varies widely among different species. The American robin both hops and walks, while the starling usually walks. The ostrich can run as fast as 64 km/h (40 mph). Swifts, however, can neither hop nor run; their weak feet are useful only for clinging to vertical surfaces, such as the walls of caves and houses.
Birds that walk in shallow water, such as herons and stilts, have long legs that facilitate wading. Jacanas, which walk on lily pads and mud, have long toes and nails that disperse their weight to help prevent them from sinking. Penguins have stubby legs placed far back from their center of gravity. For this reason, they can walk only with an upright posture and a short-stepping gait. When penguins need to move quickly, they "toboggan" on their bellies, propelling themselves across ice with their wings and feet.